A lithium secondary battery is characterized in that it is small and has a large capacity, and has been widely used as a power source for electronic device such as cellular phone and notebook computer, and has contributed to the improvement in the convenience of portable IT devices. In recent years, attention is also focused on the use of a lithium secondary battery for applications where the battery is larger, including a power source to drive a two-wheeled vehicle, an automobile, or the like, and a storage battery for smart grid.
For a lithium secondary battery, high safety in a wide range of temperature conditions is required in addition to further improvement in energy density and lifetime property wherein the battery can endure long term use. Accordingly, studies have been widely conducted on various materials and additives with respect to the composition of the electrolyte solution, which has a great influence on the long-term cycle and the safety.
In general, a carbonate-based non-aqueous solvent is used for an electrolyte solution in a lithium secondary battery. That is because the carbonate-based solvent has excellent electrochemical resistance and is inexpensive in terms of cost. As an electrolyte solution of a carbonate-based solvent, a mixed electrolyte solution containing a cyclic carbonate such as ethylene carbonate (EC) and propylene carbonate (PC) and a chain carbonate such as diethyl carbonate (DEC) and dimethyl carbonate (DMC) is usually used. The cyclic carbonate has a high dielectric constant, and therefore has a function of dissolving/dissociating a lithium salt such as LiPF6. The chain carbonate has a low viscosity, and therefore has a function of improving the diffusibility of lithium ion in the electrolyte solution.
Because the carbonate-based solvents have low flash points and high combustibility, however, there is a risk of ignition/explosion caused by overcharge or overheating. The danger is greater in the case of ignition or explosion as the battery is larger, and therefore the improvement in safety is an important problem for a large-sized battery, in particular. In addition, during long-term cycle or under high-temperature conditions, the decomposition of the solvent of the electrolyte solution, as well as the deterioration of electrodes, may occur, resulting in the reduction in the capacity, the generation of gas, and the like. The problems tend to be recognized markedly in a lithium secondary battery comprising a positive electrode for high voltage which comprises a spinel compound such as LiNi0.5Mn1.5O4 as a positive electrode active material, in particular, the battery attracting attention as to higher energy density in recent years.
As a method for solving the problems, Patent Literatures 1 to 2 disclose that an electrolyte solution containing a solvent which contains a fluorinated phosphate, and an electrolyte (LiPF6, and the like) is used. The fluorinated phosphate has a self-extinguishing function, and the electrolyte solution to which the fluorinated phosphate is added is expected to be a flame-retardant electrolyte solution. Additionally, Patent Literature 1 discloses that the battery performance may be enhanced by changing the solvent to a mixed solvent of a fluorinated phosphate, and a chain ester and/or a cyclic ester. Patent Literature 2 discloses that it is further preferred that a cyclic carbonate and a chain carbonate are mixed into the solvent as the solvent of the electrolyte solution.
In addition, Patent Literature 3 discloses that charge and discharge properties of the battery is improved by adding a vinylene carbonate compound and/or a vinyl ethylene carbonate compound to a phosphate-based electrolyte solution.
Patent Literature 4 discloses that when a fluorinated phosphate and a specific fluorine-containing solvent such as a fluorinated ether are mixed and used, the non-aqueous electrolyte solution exhibits high flame-retardant properties, and has high electrolyte-dissolving properties, and exhibits good ionic conductivity.
Meanwhile, Patent Literature 5 discloses a non-aqueous electrolyte solution secondary battery comprising an electrolyte solution which contains a polymeric phosphate having two or more phosphate groups in the molecule, together with a chain phosphate such as trimethyl phosphate, and teaches that the secondary battery has good load properties and high safety. The polymeric phosphate is used herein as an additive solvent to form a film having high lithium ion permeability on the surface of the negative electrode and thereby improve the load properties.